Fix chimney condensation before installing insert

Q: My fireplace is located in the basement, and the chimney is located on the wall between the house and attached garage. In the summer when it is very humid outside, the glass doors on my fireplace sweat and the condensation pools on the floor in front of the fireplace hearth. We keep our home cool (67 degrees) at night in the summer. If I open the glass doors, the condensation does not occur.

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Posted Mar. 17, 2012 at 12:01 AM

Posted Mar. 17, 2012 at 12:01 AM

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Q: My fireplace is located in the basement, and the chimney is located on the wall between the house and attached garage. In the summer when it is very humid outside, the glass doors on my fireplace sweat and the condensation pools on the floor in front of the fireplace hearth. We keep our home cool (67 degrees) at night in the summer. If I open the glass doors, the condensation does not occur.

I would like to put an electric fireplace insert in, but I am concerned about the condensation. The insert resembles a flat-screen TV in appearance. Any condensation could damage the insert. I have only one fireplace and it is the only thing that vents through the chimney.

I have thought about inserting insulation into the top of my chimney and capping it, but I am concerned that the walls of the flue could create condensation during the humid summer days. Do you have any recommendations?

A: Condensation in the home is the process where moisture, which is present in the conditioned air, is transformed from a gas to a liquid.

This process is most often noticed in the winter, when moisture forms on the cold glass surfaces of windows. It's a simple fact that cold air cannot hold or retain as much moisture as warm air, and when the two meet, the warm air starts to cool, releasing some of its moisture as condensation.

Homes with high humidity levels will often have condensation forming on the interior side of windows in colder weather. When the humidity levels are too high, mold and mildew can become a problem. A normal and comfortable level of humidity inside the home would be anywhere between 30 percent and 60 percent at 65 degrees.

Humidity levels below 30 percent can result in dry skin, nosebleeds and static electric buildup, whereas higher moisture levels lead to the aforementioned mold and mildew.

A hygrometer, a device that measures humidity levels, can be purchased for under $20 and will give you an accurate reading. Only when you know you have a humidity problem can you correct it.

As you know, if you plug the top of the chimney above the fireplace, the flue-liner condensation will trickle down to the fireplace. You will need to protect the insert from moisture problems. According to one manufacturer's recommendation, "Insulation and vapor barrier should be placed a minimum of 2 inches from the unit."

This means you can use rigid foam insulation behind some inserts to control temperatures and airflow and a vapor barrier to protect the insert from moisture, but first read and follow all installation instructions.